[RFC 1/2] RISC-V: clarify ISA string ordering rules in cpu.c

[Conor Dooley <conor.dooley@microchip.com>]

While the list of rules may have been accurate when created, it now
lacks some clarity in the face of isa-manual updates. Specifically:

- there is no mention here of a distinction between regular 'Z'
  extensions which are "Additional Standard Extensions" and "Zxm"
  extensions which are "Standard Machine-Level Extensions"

- there is also no explicit mention of where either should be sorted in
  the list

- underscores are only required between two *multi-letter* extensions but
  the list of rules implies that this is required between a multi-letter
  extension and any other extension. IOW "rv64imafdzicsr_zifencei" is a
  valid string

Attempt to clean up the list of rules, by adding information on the
above & sprinkling in some white space for readability.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
---
 arch/riscv/kernel/cpu.c | 22 +++++++++++++++++-----
 1 file changed, 17 insertions(+), 5 deletions(-)

diff --git a/arch/riscv/kernel/cpu.c b/arch/riscv/kernel/cpu.c
index 852ecccd8920..5e42c92a8456 100644
--- a/arch/riscv/kernel/cpu.c
+++ b/arch/riscv/kernel/cpu.c
@@ -120,20 +120,32 @@ device_initcall(riscv_cpuinfo_init);
 		.uprop = #UPROP,				\
 		.isa_ext_id = EXTID,				\
 	}
+
 /*
  * Here are the ordering rules of extension naming defined by RISC-V
  * specification :
- * 1. All extensions should be separated from other multi-letter extensions
- *    by an underscore.
+ *
+ * 1. All multi-letter extensions should be separated from other multi-letter
+ *    extensions by an underscore.
+ *
  * 2. The first letter following the 'Z' conventionally indicates the most
  *    closely related alphabetical extension category, IMAFDQLCBKJTPVH.
- *    If multiple 'Z' extensions are named, they should be ordered first
- *    by category, then alphabetically within a category.
+ *    'Z' extensions should be sorted after single-letter extensions and before
+ *    any higher-privileged extensions.
+ *    If multiple 'Z' extensions are named, they should be ordered first by
+ *    category, then alphabetically within a category.
+ *
  * 3. Standard supervisor-level extensions (starts with 'S') should be
  *    listed after standard unprivileged extensions.  If multiple
  *    supervisor-level extensions are listed, they should be ordered
  *    alphabetically.
- * 4. Non-standard extensions (starts with 'X') must be listed after all
+ *
+ * 4  Standard machine-level extensions (starts with 'Zxm') should be
+ *    listed after any lower-privileged, standard extensions.  If multiple
+ *    machine-level extensions are listed, they should be ordered
+ *    alphabetically.
+ *
+ * 5. Non-standard extensions (starts with 'X') must be listed after all
  *    standard extensions. They must be separated from other multi-letter
  *    extensions by an underscore.
  */
-- 
2.38.1